/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode * head = nullptr,*tail = nullptr;
        int carry = 0;
        while(l1||l2)
        {
            int n1 = l1?l1->val:0;
            int n2 = l2?l2->val:0;
            int sum = n1+n2+carry;
            if(!head)
            {
                head = tail = new ListNode(sum%10);
            }else{
                tail->next =  new ListNode(sum%10);
                tail = tail->next;
            }
            carry = sum/10;
            if(l1)
            {
                l1 = l1->next;
            }
            if(l2)
            {
                l2 = l2->next;
            }
        }


        if(carry>0)
        {
            tail->next = new ListNode(carry);
        }
        return head;
    }
};
